"The Incredible Convenience of Advertising on Social News Sites".
That aside, this isn't really a killer app. It is much easier to adjust an image in the Gimp or Photoshop than it is to do so in Mathematica. If you are doing image processing, it might be nice to have that available in Mathematica, but it is equally easy to transform images in your $FAVORITE_PROGRAMMING_LANGUAGE.
Don't think "image processing", think "signals processing." The ability to perform complex interactive transforms on datasets representable as images is actually pretty neat.
I doubt Mathematica lets you do anything that competing products like Matlab's image processing toolkit don't do, it just greatly enhances the level of activity associated with image processing.
That aside, this isn't really a killer app. It is much easier to adjust an image in the Gimp or Photoshop than it is to do so in Mathematica. If you are doing image processing, it might be nice to have that available in Mathematica, but it is equally easy to transform images in your $FAVORITE_PROGRAMMING_LANGUAGE.